1. Technical Field
The present invention relates to an image sensor and a method of manufacturing the same and, more particularly, to a complementary metal oxide semiconductor (CMOS) image sensor and a method of manufacturing the same.
2. Description of the Related Art
Image sensors, which transform optical images into electric signals, are generally classified into a charge coupled device (CCD) image sensor type and a CMOS image sensor type. The CCD image sensor includes a plurality of MOS capacitors for transferring free electrons created by incident photons, and the CMOS image sensor includes a plurality of unit pixels and electric circuits for controlling the unit pixels.
The CCD image sensor has high power consumption, a complicated manufacturing process and a complex operating mode. Generally, signal processing circuits are difficult to fabricate in one CCD chip. In contrast, the CMOS image sensor is relatively easy to manufacture. Recent research on the image sensors has been intensively focused on the CMOS image sensor.
The CMOS image sensor includes a pixel area in which an image is captured and a logic area for controlling output signals generated from the pixel area. The pixel area includes a photodiode and a MOS transistor and the logic area includes a plurality of MOS transistors.
The pixel area and the logic area may be stacked on a substrate, and the transistors in the pixel area and the logic area may be formed together.
To manufacture the CMOS image sensor, the photodiode is formed on a substrate and then the MOS transistor is formed on the substrate in the pixel area. A silicon nitride layer is formed on the photodiode and the CMOS transistor and is partially removed to form a gate spacer on a sidewall of a gate electrode of the MOS transistor. The silicon nitride layer remains on the photodiode after the spacer is formed. Then, an anti-reflection layer is formed on the silicon nitride layer and is partially removed to expose the photodiode. The anti-reflection layer prevents a diffused reflection from the spacer in a subsequent process.
Forming the spacer and the anti-reflection pattern typically require several photolithography processes. For example, a first photoresist pattern is formed on the silicon nitride layer through a first photolithography process, such that the photodiode portion is covered with the photoresist and the CMOS transistor portion is exposed. The silicon nitride layer is anisotropically etched using the first photoresist pattern as an etching mask, thereby forming a gate spacer on the sidewall of the gate electrode of the MOS transistor. A second photoresist pattern is formed on the anti-refection layer through a second photolithography process, such that the photodiode portion is exposed and the CMOS transistor portion is covered with the photoresist. The anti-reflection layer is partially etched using the second photoresist pattern as an etching mask, thereby forming an anti-reflection pattern on the silicon nitride layer.
As the density of pixels of a CMOS image sensors increase, there is a tendency for an electric signal generated from a first pixel area to be influenced by another electric signal generated from a second pixel area adjacent to the first pixel area, thereby deteriorating image quality. This is known as pixel crosstalk. The pixel crosstalk is high when the photons falling on one pixel are “falsely” sensed by other pixels around it. Metal wiring is formed into a multilayer structure in the image sensor such that an optical path incident to each pixel becomes longer and reduces the photosensitivity of the pixel area of the image sensor.